The present invention relates to a variable performance valve train used in internal combustion engines. More particularly, the present invention pertains to a variable performance valve train having three-dimensional cams, the profile of which continuously changes along the axis of a camshaft.
Japanese Unexamined Patent Publication No. 5-125966 discloses a first prior art apparatus, which includes a variable valve lift mechanism. The variable valve lift mechanism includes intake and exhaust valves, which are driven by camshafts, and low speed and high speed cams for driving the intake valves or the exhaust valves. The mechanism varies the valve lift, or the valve open angle, of the intake valves or the exhaust valves. In this specification, the valve open angle refers to an angle of rotation of a crankshaft during which an intake valve or an exhaust valve is open.
When there is a malfunction in the control of a throttle valve, the mechanism of Japanese Unexamined Patent Publication No. 5-125966 selects a set of cams that will decrease the engine power. Specifically, the mechanism uses either high speed cams or low speed cams such that the vehicle speed is reduced.
Instead of having two types of cams, a variable performance valve train according to a second prior art apparatus has three-dimensional cams, the profile of which continuously changes along the axis of a camshaft. However, unlike the prior art apparatus of Japanese Unexamined Patent Publication No. 5-125966, a valve train having three-dimensional cams cannot employ the valve switching control and fail-safe control for decreasing the engine power.
For example, if the mechanism having the three-dimensional cams simply decreases engine power for performing fail-safe control, engine starting is hindered or engine speed stability deteriorates.
A valve train according to a third prior art apparatus disclosed in Japanese Unexamined Patent Publication 8-177434 includes the high speed and low speed Cams of Publication No. 5-125966 and a variable valve timing mechanism. The variable valve timing mechanism adjusts the rotational phase of the camshaft.
When either the variable valve lift mechanism or the variable valve timing mechanism malfunctions, the variable valve lift mechanism uses the low speed cams, and the variable valve timing mechanism retards the rotational phase of the camshaft. This prevents the intake valves from interfering with the pistons and the exhaust valves.
A malfunction of the variable valve lift mechanism could be either a malfunction of the low speed cams or a malfunction of the high speed cams. In order to perform fail-safe control, different programs must be prepared for a malfunction of the low speed cams and for a malfunction of the high speed cams. The two fail-safe controls increase the time and effort required to make the programs and increase the required memory capacity for storing the programs.
If the variable valve lift mechanism malfunctions when the high speed cams are being used, the mechanism may not be able to Switch to the low speed cams. In this case, engine starting and engine speed stability will deteriorate.